What about Gabriel? He isn't always in sync with Brenda, and there was an earlier episode or two when his loyalty was in question. Not the part where he was the only one who didn't get a subpoena...that was just a ploy. A couple of seasons earlier than that. And he did once ask for a transfer, when his office romance turned sour. He doesn't have the clear motive Pope does, though. Maybe just plain old ambition? There's nothing in his character to suggest that, but he'd make a good "least likely suspect."